1. Field of the Invention
The present invention relates to a navigation system that is capable of searching a recommended route based upon a plurality of sets of map data at different scales and drawing the recommended route superimposed upon display maps at different scale.
2. Description of the Related Art
Vehicular navigation systems in the known art are provided with a function for displaying a roadmap of the area where the vehicle is currently located, a function for accurately detecting the vehicle position through map matching, a function for calculating a recommended route from a point of departure to a destination and the like. In these vehicular navigation systems in the prior art, roadmap display data, data for map matching and data for route search are separately stored in a CD ROM in order to maintain compatibility with existing software programs and also to improve the processing speed.
The roadmap display data include widest range map data for displaying large areas at the smallest scale, most detailed map data for displaying small areas in detail at the largest scale and a plurality of sets of map data at different scales between that of the widest range map data and that of the most detailed map data. For instance, the widest range map data may be referred to as level 4 data, the most detailed map data may be referred to as level 1 data and the sets of data at scales between those of the level 4 data and the level 1 data may be referred to as level 3 data and level 2 data. In that case, the route search data include two sets of data that correspond to the level 4 and level 2 roadmap data, and the vicinity of the point of departure and the vicinity of the destination are searched using the level 2 data and the other area is searched using the level 4 data for route search to reduce the length of time required for the route search.
In this specification, a level with a large level number is referred to as a high order level, whereas a level with a small level number is referred to as a low order level.
FIGS. 21A and 21B illustrate a roadmap depiction of roadmap display data which are stored in memory as level 4 data and level 3 data. The level 4 roadmap display data and the level 3 roadmap display data are separately stored in CD ROM media. FIG. 21A shows a roadmap corresponding to one of the meshes in the level 4 data, i.e., a mesh M4, in which a road D1 and two roads D2 and D3 that connect at intersections C1 and C2 at the two ends of the road D1 are present. One of the small areas achieved by dividing one of the meshes of the level 4 data, i.e., the mesh M4 equally into sixteen portions, i.e., the hatched small area m3, constitutes a mesh of the level 3 data, i.e., a mesh M3 and, as shown in FIG. 21B, only a portion of the road D1, i.e., a road D4 is present in the mesh M3.
There is a problem in the system in the prior art described above in that when the recommended route is displayed superimposed upon a roadmap at level 2 or level 1 displayed on the monitor based upon the recommended route data which constitute the results of a route search performed at level 4, a great length of time is required for data conversion processing.
Also, if the detail button is operated while the roadmap corresponding to the mesh M4 in the level 4 data is displayed on the monitor, the roadmap corresponding to the mesh M3 in the level 3 data will be displayed on the monitor. However, since there are no identification data to indicate that the road D1 and the road D4 are the same road, it is difficult to make a given road correspond through the different levels. In addition, a similar problem exists in regard to the route search data, and it is difficult to make the data searched at level 2 correspond to the data searched at level 4 in recommended route data that are the result of a route search. Moreover, since there are no identification data to indicate that a road in the roadmap display data and a road in the route search data are the same, it is difficult to achieve correspondence of a given road when the recommended route data are superimposed upon the roadmap display data.